It is often necessary to interrupt or divert very high currents in electrical circuits by the opening of a switch, either in the path of the current or in a parallel circuit loop. Since very high currents are typically associated with very high electrical voltages, either at the voltage source or as a result of inductance in the circuit, the opening of such a switch almost inevitably results in the creation of an electric arc between the terminals that have been disconnected. The establishment of such an arc increases the effective opening time of the switch, encourages faults in the switch to occur and can be dangerous.
Various means exist for preventing or extinguishing such electric arcs in switches, such as by air blasting, exploding wire fuses, and high explosive driven arc quenching devices. Low arcing switches carrying high currents are used in inductively stored pulse power applications to open the primary storage inductance. As regards protection devices such as circuit breakers, however, the high current arc, once formed, is very difficult to extinguish. All of the above mentioned devices have serious limitations regarding their size, speed, initial circuit loading, and the magnitude and/or duration of the switch operating resistance profile.
Explosive-driven circuit breakers and switches which utilize the power of an explosion to separate electrical contacts or to break an electrically conductive path, and sometimes to simultaneously blow away the resultant arc, are well known. U.S. Pat. No. 3,851,219, to Kozorezov et al, titled "Circuit-Opening Device For Interrupting Heavy Currents By Means of An Explosive Charge", for example, teaches the use of two hollow conductors made of a material with a low density and high thermal and electrical conductivity, e.g., magnesium or aluminum, and an explosive cartridge within each, with the two connected in parallel. When the explosive charge of one of the cartridges is set off, the hollow conductor disintegrates, and current flow therethrough is interrupted.
U.S. Pat. No. 4,342,978, to Meister, titled "Explosively-Actuated Switch and Current Limiting, High Voltage Fuse Using Same", discloses the use of an explosive to drive apart two earlier contacting electrical contacts by means of a piston which is made of a material that ablates to produce gases which extinguish the electric arc that results.
Other devices in which one or two gaps generated by an explosion are shielded by means of arc extinguishing gas or by containment within an insulating element are exemplified by U.S. Pat. No. 4,370,531, to Tobin, U.S. Pat. No. 4,472,704, to Jackson et al, and U.S. Pat. No. 4,490,707 to O'Learyl.
In "Staged Explosively Driven Opening Switch Development for Explosive Flux compression Generators", by Levi et al published by the Air Force Weapons Laboratory, the by-products of the explosion are directed across the arc formed when a circuit is disrupted by an explosive-activated switch to extinguish the arc. A switch of this kind, wherein an aluminum conductor carrying a current is broken apart by an explosive cord is described in "High Recovery Voltage Switch for Interruption of Large Current", Ford et al, Rev. Sci. Instrum. 53(7), July 1982.
SF.sub.6 is used for arc interruption in high current switches in "Advances in High Voltage Insulation and Arc Interruption in SF.sub.6 and Vacuum", by Maller et al, Pergamon Press at chapter 4, pages 100-108, and the use of gas with or without an explosive to effectuate separation of high current electrical contacts is described in U.S. Pat. No. 3,748,418, to Kawasaki, titled "Tank-Type Gas-Filled Circuit Breaker With Impulsive Seal Breaking Means for Initiating Piston Operation".
As best seen in FIG. 1, a simple stored power circuit 10, of the type common to pulse power operations, consists of two loops with one comprising a power storing capacitance 14 connected generally in series with a storage inductance 15 and a switch 12 to provide a primary conductive path for a current A when the switch is closed. The second loop connected in parallel with switch 12, comprises a switch 11 and a load 13, such that if switch 11 is closed and switch 12 is open, a current B will flow through the load 13.
Assume that capacitor 14 is initially charged by a voltage source (not shown). To obtain a power pulse through the load 13, switch 12 is initially closed and switch 11 is open establishing only current loop A. Then, at a predetermined time, switch 12 is opened and, thereafter, switch 11 is closed, thereby directing current from the primary circuit to the load 13. Because the opening of switch 12 will result in the formation of an electric arc due to inductance 15 in the circuit, the application of current to the load 13 by the closure of switch 11 is somewhat delayed. It is therefore desirable, for high speed switching, to have an arcless driversin of the primary current through the associated secondary part of the circuit.
Although there are numerous switching devices to obtain current interruption and extinguishment of the resultant arc, there exists a need for a method and apparatus for controllably and rapidly increasing the electrical impedance of an opening switch element in inductive type high power pulse generating circuitry to provide safe and reliable power pulse generation.